Looked into this... surprisingly did not find an app to do this *from* Giphy on iOS, but I always end up going to Giphy's Caption tool on web when I need to add a caption: https://giphy.com/create/gifcaption
Clippy is very good when you wanna make a GIF from a YouTube video and add whatever caption you like (on Mobile) https://www.producthunt.com/post...